It was discovered recently that bright light affects human physiology and behavior in ways that ordinary indoor light cannot. Bright light has proven to be an effective antidepressant for winter depression (seasonal affective disorder), and may have benefit in other affective disorders as well. Thus, basic research about bright light may increase our understanding of the affective disorders. Some research has suggested that bright light can be used to phase-shift human circadian rhythms, and that the direction and amount of the shift depends on when the light is applied within the circadian cycle. If we learn how to use bright light to shift rhythms, then we will be able to drive practical applications for shift- work, jet-lag, and some sleep disorders. These are the long-term objectives of the present proposal. Three experiments are proposed in which the sleep schedule is shifted and bright light is used to help the circadian rhythms adapt to the shifts. One experiment uses gradually shifting sleep schedules which are the equivalent of 26, 27 and 28-hr days. Another uses 26 and 22-hr schedules. The third test abrupt advance and delay shifts of about four hrs. Body temperature measurements provide a marker of the internal circadian rhythms. Questionnaires are used to assess the actual time of sleep, sleep quality, fatigue, and mood. Subjects live at home and carry on their usual activities throughout the experiments. Thus they are exposed to 24-hr time cues which may conflict with the shifting sleep schedules. The results of these experiments, therefore, will apply more directly to the practical applications. A fourth experiment has the methodological focus of how best to obtain stable overnight baseline temperature recordings. The aim is to develop methods that can be used by other researchers. The fifth study is to test light- work-sleep schedules on nurses who are shift workers. These schedules make use of what is already known about shifting circadian rhythms with bright light. As more basic knowledge becomes available from the three experiments proposed on shifting circadian rhythms, the nurse schedules will be expanded and modified accordingly. The aim of the light-work-sleep schedules is to improve sleep and reduce fatigue in nurses who rotate to the night shift. If the nurses benefit as predicted, then patient care and safety should be improved.